1. Field of the Invention
The present invention relates generally to a fuel injection control system for a multi-cylinder internal combustion engine, such as an automotive engine. More specifically, the invention relates to so-called sequential fuel injection control system which performs fuel injection for one of a plurality of engine cylinders independently of other engine cylinders at a timing around the induction top-dead-center (TDC) thereof so that the fuel injected an air/fuel mixture to be introduced into the corresponding cylinder. Further, the invention relates to an acceleration enrichment technique for the sequential injection type fuel injection system for the internal combustion engine.
2. Description of the Prior Art
Sequential injection type fuel injection control have been known as one of desirable fuel injection technique since it can allow precise fuel injection amount control depending upon the engine driving condition. Especially, such a type of fuel injection control has been regarded as beneficial to employ since it may exhibit better response characteristics in fuel injection amount control relative to variation of the engine load.
For independent fuel injection control for each of a plurality of engine cylinders in a multi-cylinder internal combustion engine, fuel injection valves or injectors are provided for injecting a controlled amount of fuel into respectively corresponding induction branches of an intake manifold of the induction system of the engine. Each of the fuel injection valves is controlled by a fuel injection pulse which has a pulse duration corresponding to the amount of the fuel to be injected. Namely, the fuel injection valve opens in response to the leading edge of the fuel injection pulse and closes in response to the trailing edge of the fuel injection pulse.
In conventional fuel injection timing control, the fuel injection start timing, i.e. the fuel injection valve open timing has been controlled relative to a crank shaft angular position so that fuel injection can be performed across the induction TDC, i.e. about the intake valve open timing. The fuel injection valve closing timing, in conventional fuel injection timing control, is determined depending upon the fuel injection amount and in relation to the fuel injection start timing.
In sequential fuel injection control, since substantially current fuel injection control parameters, such as an engine speed N and an air flow rate or an engine load Q, can be used, precise fuel injection amount control is possible, in comparison with single-spot fuel injection or group cylinder injection which performs fuel injection for more than one engine cylindes. However, since fuel injection is maintained across the induction TDC where the intake valve opens, only part of the fuel collides into the valve head of the intake valve and a thus atomized to establish a uniform mixture rate of air/fuel mixture. Remaining fuel is maintained in a state not satisfactorily atomized and is introduced into the engine combustion chamber in the relatively low temperature liquid state. Therefore, the temperature of the air/fuel mixture to be introduced into the combustion chamber becomes relatively low. This lowers combustion efficiency and propagation in the combustion engine. Thus, the engine performance and drivability tends to be degraded at certain engine driving conditions where a relatively large amount of fuel is required for causing expansion of the fuel injection pulse width over the intake valve open timing.
In order to avoid such defects, the Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 59-29733 discloses a sequential fuel injection control technique. In the disclosed fuel injection control, the timing of the fuel injection pulse is determined so that the trailing edge of the fuel injection pulse should not be delayed after the induction TDC of the corresponding engine cylinder. Therefore, the timing of the leading edge of the fuel injection pulse is varied depending upon the required fuel injection amount. In this case, fuel injection can always be performed between the induction TDC to sufficiently heat the air/fuel mixture for better engine performance and drivability.
However, on the other hand, in such fuel injection control, since the fuel injection pulse is set to close the fuel injection valve at a fixed crank shaft angular position before the induction TDC of the corresponding engine cylinder, the pulse duration should be fixed after starting fuel injection. In other words, in this type fuel injection control, the fuel injection control parameters which can be used for actual fuel injection control cannot be the substantially current data but are necessarily the instantaneous data immediately before the occurrence of the fuel injection pulse. This clearly degrades response characteristics of the fuel injection control relative to the engine load variation.
In addition, when substantial change in engine acceleration occurs during fuel injection, the air/fuel mixture to be established becomes too lean to degrade the engine acceleration characteristics.
In order to improve such a defect in the prior proposed system, there is another proposal in the Japanese Patent First Publication (Tokkai) Showa No. 63-9646 which corresponds to the co-pending U.S. patent application Ser. No. 067,007, filed on June 29, 1987, now abondoneds and which proposes to fix the fuel injection terminating timing at the crank shaft angular position corresponding to the induction TDC of the corresponding engine cylinder. This allows all of the fuel injected to collide onto the valve head of the intake valve and to become satisfactorily atomized for establishing a substantially uniform air/fuel mixture. In the proposed system, the duration of operating the fuel injection valve is expanded to compensate the fuel amount required by the increase of the engine load when the engine load is increased during fuel injection. This is successful in preventing the air/fuel mixture from becoming too lean.
However, a problem is encountered due to delay of response in an air flow meter which measures the induction air flow rate. Namely, the air flow rate indicative signal tends to indicate a smaller air flow rate than the actual amount at the initial stage of increase of the engine load. Since the expansion of the fuel injection pulse duration is determined according to the air flow rate indicative signal, the smaller air flow rate indicative signal value results in a smaller expansion rate of the fuel injection valve duration. This makes the engine acceleration characteristics not precisely in according with engine acceleration demand.